ABSTRACTSerum levels of IgE, other immunoglobulins and C'3 were measured in 36 MS patients, and the results compared with those of 40 age‐ and sexmatched patients hospitalized on Neurology Service. Diagnoses among controls included cerebral infarction, cervical spondylosis, nonmigrainous headache, seizure disorders and peripheral neuropathy. Six patients in the MS group and seven in the non‐MS group had a past history of allergy to food, drugs, dust or other substances. IgE levels were measured by double‐antibody radioimmunoassay. Other immunoglobulins (G, M, D and A) and C'3were quantitated by Mancini's method. Results indicate that the median IgE and C'3levels of MS patients were slightly lower than those for non‐MS subjects. Concentrations of other immunoglobulins were similar for the two groups. CSF samples from 12 MS and five non‐MS patients were studied and none of these contained measureable (>7.5 U/ml) IgE. In view of:1) recent‐reports deseribing mast cells in MS plaques, 2) the suggested role of biogenic amines in the pathogenesis of demyelinating diseases, and 3) because IgE exists in free and cell bound state, it is suggested that tissue surveys of MS plaques for IgE deposits similar to those seen in glomeruli in nephrotic syndromes may provide a clue to the pathoge